The present invention relates to the cooling of hot particulate solids and, more particularly, to the cooling of hot particulate material drained from a fluidized bed prior to transporting that particulate material away from the bed with conventional mechanical conveying equipment.
Fluidized bed combustors have long been recognized as an attractive means of combusting particulate fuel to generate heat. In a typical present day fluid bed furnace, particulate fuel, typically coal having a top size ranging from about 3.0 to about 6.5 mm, is fed to and combusted within a fluidized bed of similar sized particulate material at a temperature in the range of about 760.degree. C. to 925.degree. C. The particulate material comprising the bed usually contains in addition to the particulate fuel, a particulate sulfur absorbent, most commonly limestone, ash material from previously combusted fuel, and sulfated absorbent particles. Fluidizing air, which also serves as combustion air, is supplied to the fluid bed from an air plenum located beneath the bed support plate. The fluidizing air passes upwardly from the air plenum into the fluidized bed through a plurality of holes in the bed support plate at a flow rate sufficiently high to fluidize the particulate material within the fluid bed without entraining a significant portion of that material in the flue gases generated within the bed.
In such fluidized beds, typically referred to as bubbling beds, only a small portion of the particulate material, that particulate material which is relatively fine compared to the majority of particulate material in the bed, is entrained in the flue gases generated within the bed. Such entrained material is removed from the flue gas in a mechanical collector disposed downstream of the furnace prior to venting the flue gas to the atmosphere. As only a small portion of the particulate material is entrained in the flue gas and removed from the bed in such a manner, particulate material tends to accumulate within the bed and must be periodically or continuously drained from the bed. To accomplish this, it is customary to provide drain pipes which extend upwardly through the bed support plate into the bed to receive particulate material therefrom. The particulate material draining from the bed passes through the bed drain pipe to a conveying means disposed beneath the fluidized bed furnace, typically a mechanical screw conveyor or other type of mechanical feeder capable of transporting the material from the bed drain pipe outlet to a location external of the furnace for recycling or disposal.
As combustion occurs within the bed at temperatures in the range of about 760.degree. C. to about 925.degree. C., the particulate material draining from the bed through the bed drain pipes is very hot and, unless cooled, may damage the mechanical equipment used to transport it from the bed drain pipe outlet. Therefore, unless more expensive water-cooled conveying equipment is utilized, frequent maintenance and replacement of the conveying equipment are required.
Due to this fact, it has been customary in the industry to attempt to cool the particulate bed drain material somewhat prior to admitting the bed drain material to the conventional transport equipment disposed beneath the bed. One manner in which to cool the particulate material is to pass a stream of cooling gas, most commonly air, upwardly through the bed drain pipes in counterflow and in direct contact with the particulate material draining through the pipe. The pressure of the cooling gas is controlled such that the velocity of the cooling gas passing upwardly through the drain pipe does not hinder the fall of bed drain material through the pipe. In this manner, heat is transferred directly from the particulate material to the cooling gas flowing upwardly through the bed drain pipe. An alternative method for cooling the bed drain material involves passing the bed drain material over a heat exchange coil disposed within the bed drain pipe. A cooling fluid, typically water, is passed through the heat exchange coils in indirect heat exchange relationship with the hot particulate solids flowing over the external surface of the heating coil. A fluidized bed unit incorporating a bed drain conduit wherein the particulate solids are cooled by direct contact with an upwardly flowing cooling gas and also by indirect contact with a cooling fluid flowing through a heat exchange coil disposed in the bed drain conduit is shown in U.S. Pat. No. 4,227,488.
Cooling systems of this type, however, have disadvantages. One disadvantage is that the presence of a heat exchange coil within the bed drain conduit can result in the hindering of the flow of bed drain material through the conduit and result in pluggage of the bed drain conduit. The use of an upwardly flowing cooling gas in direct contact with the downwardly passing bed drain material will necessarily slow the rate at which bed drain material can be removed from the bed and also result in the entrainment of some small particle bed drain material in the cooling gas. The presence of this particulate material in the cooling gas limits the uses which can be made of the cooling gas and, therefore, the cooling gas must generally be admitted to the fluidized bed chamber so that any particulate solids therein will not be vented to the atmosphere.
It is an object of the present invention to provide a simplified apparatus for cooling hot particulate solids drained from a fluidized bed combustor.